Oswald Achenbach (2 February 1827 – 1 February 1905) was a German painter associated with the Düsseldorf school of painting. Though little known today, during his lifetime he was counted among the most important landscape painters of Europe. Through his teaching activities, he influenced the Kunstakademie Düsseldorf. His brother, Andreas Achenbach, who was twelve years older, was also among the most important German landscape painters of the 19th century. The two brothers were humorously called "the A and O of Landscapes" (a reference to their initials matching a common German reference to the Alpha and Omega).